A. Field of Invention
This invention pertains to a pacemaker that automatically monitors the capability of an impaired heart to increase cardiac output with increased rate and to optimize the AV delay in such a way that the ability to increase cardiac output with rate is maximized. To enable this function to be carried out, it is suggested to use the integral of the evoked electrical depolarization as an indicator of the change in the cardiac volume that leads to a modulation of cardiac output.
B. Description of the Prior Art
The primary function of a dual chamber pacemaker is to monitor the occurrence of atrial events (P waves) and on the detection of these events, to set up an atrio-ventricular delay (AV delay). The ventricle is paced at the end of this delay unless a ventricular event is sensed. If atrial events are not sensed within a specified time period (standby interval) an atrial pacing pulse is generated and subsequent: ventricular pacing pulse is applied after the specified AV delay. It is common to have an AV delay after a P wave which differs from an AV delay after an A pace. The reason for this feature is that the atrial electrode is positioned away from the direct path of the depolarization traveling from the sinus node to the ventricle. In general, the AV delay after an A sense is shorter than that after an A pace.
The actual cardiac output, in particular at rest, is often influenced by the AV delay. This implies that optimum function of the heart is achieved with optimum AV delay which is different for each patient. Prior art references such as U.S. Pat. No. 4,303,075 and 5,330,511 discuss the automatic adjustment of the AV delay based on certain hemodynamic or metabolic indicators. All these known concepts assume that the optimization of the AV delay at a particular rate allows correct optimization for all rates and they do not take into consideration the possibility that at different rates (in particular higher rates) the AV delay has to be dramatically changed because of underlying pathologies that are not present in normal hearts. The mechanisms that come into play with impaired hearts that need particular attention are the regurgitation of valves, in particular the mitral valve, and the change in the ability of the ventricle to pump at higher rates because of the additional stress to an already overloaded heart. These factors imply that what is needed is to establish that the increase in rate that is demanded of the heart can in fact increase cardiac output with an optimized AV delay.
The invention is preferably implemented in dual chamber pacemakers in combination with other features such as rate response, maximum rate controls, AMS and so on.